Certain signal processing techniques involve the simultaneous detection of a plurality of analog information signals for the purpose of acquiring data represented by the signals. For example, certain commercially available medical imaging systems such as CT scanners are used to image internal features of an object under view by exposing the object to a preselected amount and type of radiation. Detectors sense radiation from the object and generate analog signals representative of internal features of the object.
In the example of CT scanners, those of the third generation type include an X-ray source and X-ray detector system secured respectively on diametrically opposite sides of an annular-shaped disk. The latter is rotatably mounted within a gantry support so that during a scan the disk continuously rotates about a rotation axis while X-rays pass from the source through an object positioned within the opening of the disk to the detector system.
The detector system typically includes an array of detectors disposed as a single row in the shape of an arc of a circle having a center of curvature at the point, referred to as the "focal spot," where the radiation emanates from the X-ray source. The X-ray source and array of detectors are all positioned so that the X-ray paths between the source and each detector all lie in the same plane, referred to as the "slice plane" or "scanning plane", normal to the rotation axis of the disk. The X-rays that are detected by a single detector at a measuring instant during a scan is considered a "ray. " Because the ray paths originate from substantially a point source and extend at different angles to the detectors, the ray paths resemble a fan, and thus the term "fan" beam is frequently used to describe all of the ray paths at any one instant of time. The ray is partially attenuated by all the mass in its path so as to generate a single intensity measurement as a function of the attenuation, and thus the density of the mass in that path. Projection views, i.e., the X-ray intensity measurements, are typically done at each of a plurality of angular positions of the disk.
In fourth generation CT scanners the detection system comprises a circular array of detectors secured on and at equiangular positions around the gantry support, equidistant from the rotation center of the disk so that the source rotates relative to the detectors. A fan beam is defined as the ray paths from the rotating source to each detector where the point of convergence of each fan beam is the corresponding detector.
The detectors used in CT scanners are usually either of the solid state type, such as cadmium tungstate detectors each having a scintillation crystal or layer of ceramic material and a photodiode, or of the gas type, such as Xenon detectors. The X-ray source can provide a continuous wave or a pulsed X-ray beam.
An image reconstructed from data acquired at all of the projection angles during a scan of both types of machines will be a slice along the scanning plane through the object being scanned. In order to "reconstruct" or "back project" a density image of the section or "slice" of the object in the defined scanning plane, the image is typically reconstructed in a pixel array, wherein each pixel in the array is attributed a value representative of the attenuation of all of the rays that pass through its corresponding position in the scanning plane during a scan. As the source and detectors rotate around the object, rays penetrate the object from different directions, or projection angles, passing through different combinations of pixel locations. The density distribution of the object in the slice plane is mathematically generated from these measurements, and the brightness value of each pixel is set to represent that distribution. The result is an array of pixels of differing values which represents a density image of the slice plane.
While the signals generated by the detectors through the series of readings provide the required data to generate the 2-dimensional image, acquiring and processing the data can pose various design problems. For example, a large number of detectors must be used for each set of readings taken for each projection view, and a large number of projection views must be taken during a scan in order to create a detailed image with sufficient resolution (a typical third generation CT scanner contains on the order of 350 to 1000 detectors, with, for example, 600 to 3000 projection views being taken within a period of 2 seconds resulting in data values, i.e., detector readings, on the order of one million, although these numbers can clearly vary). The resolution of the image created can be improved by increasing the number of detectors used and/or sets of readings, i.e., projection views, utilized. This increases the amount of data acquired and, therefore, the amount of signal information that must be processed. Accordingly, with over approximately one million data values acquired during a typical CT scan the analog signals acquired in each set of readings or views must be quickly and efficiently digitized so that computer processing can be utilized to provide relatively fast results.
Thus, in order to process the data received from the array of detectors, a data acquisition system (DAS) is used to process the data through multiple channels substantially all at the same time. The DAS includes means for converting the plurality of sets of data received from the detectors as analog signals during each projection view into corresponding digital signals so that the latter can be processed by a digital signal processor (DSP). However, various problems exist with respect to current DAS designs. For example, many DASs required for CT scanning require high digitization resolution on the order of one million (10.sup.6) to one or better, i.e., 20 bits or more. While many A/D converter techniques are known, some, such as successive approximation A/D conversion, provide inadequate signal resolution and therefore are incapable of achieving a digital signal of 20 bits or more. In this regard A/D converters using integrators have been designed to provide the required high resolution.
Where DASs are used with a continuous wave X-ray source, any modulation in the X-ray source during a scan over time will create errors. Problems are also encountered when the DAS is used with a pulse X-ray source. For example, artifacts due to variable afterglow readings of X-ray pulses are not necessarily treated identically for all of the channels. These interpulse values have an overall effect on the values of the detected analog signals corresponding to the detected X-rays in response to the pulses of X-rays from the source, and the interpulse values should be taken into consideration to provide accurate readings. In addition current leakage of certain storage devices, disposed in each channel, for storing temporarily stored information can create errors in the signal conversion.
While some of these problems can be overcome by using a separate A/D converter for each channel, until recently such an approach has been impractical because of its prohibitive cost. With the dynamic range of the analog signals provided in each channel on the order of 10.sup.6 to 1, a linear ramp A/D converter is also impractical. One DAS which overcomes or at least minimizes many of the above problems is described in U.S. Pat. No. 5,138,552, patented in the name of Hans J. Weedon and Enrico Dolazza, issued Aug. 11, 1992 and assigned to the present assignee (the "Weedon et al. Patent). The latter patent describes a DAS using non-linear digitization intervals by employing a non-linear ramp A/D converter.
In addition, CT scanners use detectors providing low level output currents. In general solid state detectors each include a layer of scintillator crystal or ceramic material for generating low energy photons as a function of high energy photons received from the X-ray source. A photodiode is provided with each scintillator crystal for generating a current as a function of the low energy photons emitted by and detected from the corresponding scintillator crystal or ceramic material. Since the photodiodes provide a low level current, a preamplifier, in the form of a transimpedance amplifier, is typically provided to convert the current to a voltage at an appropriate level so that it can be converted to a digital signal. In fact in some CT scanners using gas detectors, similar transimpedance amplifiers are utilized for the same reasons. Analog filtering of the output of each transimpedance preamplifier, prior to A/D conversion, is carried out to suppress the out-of-band portions of the wideband noise originating within the preamplifiers and the photodiodes that precede them. Doing this filtering before A/D conversion reduces the noise generating an alias within the band of frequencies containing the information data during A/D conversion. Customarily, sample and hold circuitry is provided before each A/D converter to hold each successive sample throughout the time period needed to complete the A/D conversion.
In certain CT scanners the preamplifiers and filters are apportioned among subgroups of detectors and filters and the analog signal outputs of each subgroup of detectors and filters are analog multiplexed prior to being converted to a digital signal. But analog multiplexing creates difficulties in matching the conversion characteristics of the A/D converters for the various subgroups owing to the need for a very high number of bits of resolution in converter output signals in order to reconstruct the image. Differences in conversion characteristics can cause noticeable "banding artifacts" in the final image. The banding artifacts appear as intensity variations with the reconstructed image with considerable lower spatial frequency so that they are usually noticeable. Selecting photodiodes which are physically spaced from one another for each group can reduce these artifacts. However, this will increase the likelihood of high spatial frequency components of these artifacts appearing in the image. These high spatial frequency components may also be filtered with a low pass filter, if desired, with a loss of only of some high-spatial frequency detail in the final image.
Further, spaced apart detectors for each subgroup leads to complex interconnections complicating data transfer. In addition spaced apart detectors using a time multiplexed architecture increases the physical distance between some of the detectors and the respective preamplifiers, increasing the chances of pick-up of extraneous electrical signals as noise.
A DAS having high resolution that has been developed for CT scanners that overcomes or at least reduces the effects of these problems is a DAS using delta-sigma oversampled A/D converters, described in U.S. Pat. No. 5,142,286 issued in the names of David B. Ribner and Michael A. Wu for Read-out Photodiodes Using Delta-sigma Oversampled Analog-to-Digital Converters (the Ribner et al. Patent). The Ribner et al. Patent describes a high-resolution A/D signal converter using components commonly used to process audio signals for use in processing data from a CT scanner. Conversion is provided through the use of oversampled, interpolative (or delta-sigma) modulation followed by digital low-pass filtering, typically using an finite impulse response (FIR) filter, and then by decimation. "Oversampling" refers to operation of the modulator at a sampling rate many times above the signal Nyquist rate, whereas "decimation" refers to subsampling so as to reduce the sample rate to the Nyquist rate. The ratio R of the oversampling rate to the signal Nyquist rate is designated the "oversampling ratio". As described in the Ribner et al. Patent, delta-sigma A/D converters having single-bit quantizers in the overall feedback loops of their delta-sigma modulators can have very linear predictable conversion characteristics so that matching the conversion characteristics of a plurality of delta-sigma A/D converters can be easily accomplished by so designing them in the same way. This result makes it feasible to use such a converter with each photodiode and preamplifier combination of a CT scanner, without the need to time multiplex in the analog domain.
The design proposed in the Ribner et al. Patent, necessarily requires a separate transimpedance preamplifier for generating an analog output signal responsive to the photocurrent of the corresponding photodiode of the solid state detector. The analog output signal is accompanied by wideband noise. Each analog output signal is applied to an analog, anti-alias, lowpass filter, whose output is provided to the input of a corresponding delta-sigma A/D modulator of an A/D converter. The converter includes a data rate decimator and digital filter which suppresses quantization noise from the delta-sigma modulator portion of the A/D converter, as well as a component arising from remnant wideband noise from the preamplifier.
While the use of delta-sigma modulators for A/D conversion in a DAS, as proposed in the Ribner et al. Patent, provides certain advantages over the prior art A/D converters using integrators, the design proposed by the patentees has certain drawbacks. For example, there is currently great interest in decreasing the overall cost of CT scanners. The DAS significantly contributes to that cost. While the oversampling delta-sigma modulator and data rate decimator and digital filter as an A/D converter easily lend themselves to integration fabrication techniques, the transimpedance pre-amplifier and anti-alias low-pass filter do not. Currently, such analog circuitry would be expensive to fabricate as a part of an integrated chip set including the delta-sigma modulator, probably more expensive than using discrete components based upon current integration techniques. Providing a separate transimpedance preamplifier and analog filter for each detector in discrete form as the front end of each channel of a DAS, nevertheless adds significant cost to the DAS where, for example, the number of channels needed are on the order of 350 to 1000 channels. It is desirable therefore to simplify the front end of the DAS so that it can be made entirely as integrated circuitry so as to reduce the cost of the DAS.
In addition a design tradeoff exists between a DAS having a spectral response optimized for the frequency domain and a DAS optimized for the time domain. More particularly, when sequentially reading the output of a channel, it is clear that the reading during each sampling interval should be as independent as possible from the previous readings taken from that channel, as well as readings taken from the other channels. This effects the time domain properties of the A/D converter. Any "spill-over " or " cross talk" of a signal in the channel from a prior sampling interval (sometimes referred to as "view-to-view cross talk"), thus will have a negative effect on the time domain properties of the converter. On the other hand, the frequency response of the converter largely determines the signal-to-noise ratio (S/N) and thus the quality of the signal processed through the channel. The DAS using a low pass analog filter, such as a best estimate filter of the type described in U.S. Pat. No. 4,547,893 issued Oct. 15, 1985 to Bernard M. Gordon and assigned to the present assignee, for shaping the spectral response of the output of the transimpedance amplifier is optimized for its frequency domain characteristics, at the cost of some of its time-domain properties, with spillover of as much as 25% not being unusual (i.e., 25% of the signal is from previous readings in the channel). On the other hand, an integrator type analog filter, is optimized for its time-domain characteristics, since the integrator is cleared or nulled after each sampling interval, before the next sampling interval. This insures little or no spillover, however, at the cost of negatively impacting its frequency-domain properties with a substantial amount of high frequency noise being present.
In addition to the foregoing, electronic noise can be a significant problem in DASs used for CT scanners, particularly at low level detector signal levels. The design described in the Ribner et al. Patent uses a delta-sigma modulator and FIR digital filter. The noise levels of the design tend to remain substantially the same throughout the dynamic range of the input signal. Further, the noise level of X-ray flux is not substantially constant for all levels of flux, but instead is approximately proportional to the square root of the number of photons present. Thus, where the electronic noise of the circuit is substantially at a relatively fixed level, the S/N level of the analog signal provided in each information transmission channel (which is a function of both the noise level of the X-ray flux and the electronic noise), prior to digitization, tends to worsen as the signal gets smaller.
Finally, DC stability is of great concern in CT scanner DAS applications since drift of the detected signal can seriously effect the uniformity of channel to channel performance, and thus the quality of the image reconstructed from the data derived from the acquired signals.